Transistor saturation control



Filed Nov. 5, 1956 FIGJ iai f:

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TRANSISTORSATURATION CONTROL John L. Anderson, loughkeepsie, N.Y. assignor to Inter! national Business Machines" Corporation, 1N ew York,

N.Y., a corporation of-New York,

Filed .Nov. 5, 195-6, Ser. No. 620,526

4 Claims. c1. 307 "s's.'s

This .invention relates to a phenomenon known as-current saturation intransistor circuits :and: in-particular to a means for controlling the transistorxcircuitrtoprevent such saturation. V

In thedesignand development of transistor circuits wherein high output power is desired, it has frequently been the practice-in the art to operatethe transistor in such circuits at-a;point beyond ;tlie linear portion of the output characteristic. Under these conditions non-linean ity of performance and .turn-ofi delayareencountered. The condition resulting fromtheoperation beyondthe linear region of the output characteristic of a transistor is known as saturation. i

Thephenomenon known'as saturation takes place when the. number ofcarriers present inthe base-region of a transistor becomes greater than the number of carriers that can pass through the collector junction. :In azcircuie saturation is indicatedby-the fact thatthe potential-at thecollector connection approaches more nearly1the:po--

tential of the emitter connection than doeslthat of the base. number of carriers in the-base;zregioniof the"transistor produces a delay in turn olftimeiof the'transistor. This In'such a condition the zpresence ofthe excessdelay=may-bekas much as the: carrier. lifetime; of thesecarriers. Thus, it can be seen .in higlr frequency appli'cae tions, gas thefrequency goes up, this timesdela-y may be come an appreciable traction ofthe period of the signal being handled by the'circuit:

. The circuit of this invention permits normal' operation to a :point just short of saturationwand, when the-- desired pointisreached, provides a source ofBpoteIIt-itil'at the'base of the transistor'equ'al or slightly more'positivethanthe collector potential thereby establishing equilibrium and stabilizing operation at that point. p I I The primary object of this invention is to provide 'a transistor circuit'having a saturation control;

Another-object of this invention isyto'fprovide .a'transiston circuit wherein the 'potentiahof the, collector is, pre

vented fromfmore closely approaching tha t of'tlre,-.emitter.

than the base potential.-.

Stillanother. object ofithis invention-is tofprovide ja'.

transistor control circuit which automatically controlsthe current gain of the. transistor with respect to I'the currentpassing through the transistor.

Otherobjects ofthe invention will be pointed out in; the following descriptionand claims: andiillustrat'ed in:

the accompanying'drawings, which disclosegbwway Of'BX-Z Y ample, the principle oftheinventioneand the best mode; which has been contemplatednofaapplying that-principle;

-In the'drawings: j

Figure 1 is an embodiment 20f'aitransistor inverter ecircuit illustrating the control principle-of this invention.

Figure 2 is .a volt-ampere output characteristic-offthe transistor-'1 of Figure 1.

Referring now to Figure ia ?ansistor inverter. fcircuit' isshown having a transistor 1 illustrated as a PNP junction transistor, having an emitter 2, a base 3 and a col-.-

lector 4 separated by junctions 5 and 6 respectively,

The emitter 2 is-connected to reference potential or,

ground. The collector 4 is connected through a suitable load impedance shown as resistor 7 to a source of nega tivepotentialshown as battery 8. The, positive terminal of battery 8 is connected to reference potential. An output terminal 9 is provided and connected to the collector 4 of transistor 1 for signal transfer purposes well known in the art. An input terminal 10 is providedand connected through a decoupling resistor 11. and pulse connected between the anode of impedance l5 and the collector 4 of transistor 1. A. diode 18 is connected to a source of negative potential shown as battery 19 to establish adefinite 01f potential level at terminal 9.

In operation, the circuit of Figure l serves-as a conventional inverter circuit of the type disclosed in copending application,- serial number 459,322, filed September 30, 1954, Patent No. 2,891,172 issued June 16, 1959, and assigned to the assignee of this application.

In such a-circuit the potential of the output terminal.

9 inthe no signal condition approaches the potential of battery 8 and when an input signal is impressed on the base. 3', current flows through the transistor 1 and an inverted, amplified, reproduction of the input signal appears at terminal 9. In the no signal condition, in the embodiment illustrated in Figure 1 the transistor 1 presents a high impedance to current flow therethrough by virtue of the fact that the base 3 ,is held at positive potential by being connected through'resistor 14 to battery 13, thereby reverse biasing junction 5. Under these conditions the potentialat terminal 9 would approach the potential level of battery-8 but is established at the potential level of battery 19. When an input signal is applied to terurinal-10 thev potential at the base 3 drops to a value suflicient to cause the base 3 to become negative with respectto the emitter 2.- The reverse bias on junction 5 is then-overcome -permitting:current flow through transistor 1so that the potential drop across resistor 7, as the current therethroughincreases, causes the potential level at thejcollector'4, and, consequently, at the output terminal 9,, to rise.

Transistor 1 is a transistor with a high base to collector impedance from emitter 2 to collector 4 of transistor 1 is: for all practical purposes negligible in respect to the other parameters inthe current path from reference potential at the emitter 2,to the. negative potential of battery 8,-sothat the potential of collector 4 may approach reference potential or ground .as illustrated.- Thebase 3, on the.

other 'hand, due;to the magnitude. of a signal appliedat input trminallt) may-approach moreclosely theyalue off-l 1 Patented. Aug. 2; 1960 3 this signal and assume a potential farther from the emitter 2 than is that of collector 4. When this takes place a greater quantity of minority carriers are injected into the base 3 by the emitter junction 5 than can be handled by the collector junction 6. The excess of these carriers.

ance 15 and resistor 17 connected in series between the base 3 and a sourceof positive potential such as battery 13 and the anode of the asymmetric impedance 15 being connected through a resistor 16 to the collector 4 of transistor 1. Under the conditions as the current through the collector 4 of transistor 1 increases, the potential at the collector 4 is applied through resistor 16 to the anode of asymmetric impedance 15. Under the cut off condition and all conditions of operation up to saturation the base 3 is at a potential with respect to the anode of impedance 15 that impedance 15 is then in its high impedance condition and is effectively cut 013?. As the current through the collector builds up, an equilibrium point will be reached between the potential at the base 3 and the potential at the collector 4; since the potential at the collector 4 is applied to the anode of impedance 15 at this point, impedance 15 changes to alow impedance condition' In this low impedance condition the positive potential of battery 13 is now applied to the base 3. This in effect reduces the magnitude of the negative potential at the base 3 and prevents a difference in potential between the base 3 and the collector 4. Reduction in magnitude of the potential at the base 3 reduces the base to collector amplification factor (a) of the transistor 1 so that saturation is prevented.

It will be apparent that the choice of values of resistors 16 and 17 will govern the point in the output characteristic at which saturation will be prevented. Referring to Figure 2 a current-voltage output characteristic of transistor 1 in the embodiment illustrated in Figure 1 is shown wherein for the no signal condition, the current through transistor 1 is essentially zero departing therefrom by the value of the back current through the collector junction. A load line is shown, the slope of which is de termined by the impedance in the collector circuit. When the transistor is conducting at voltage V and current I an increase of base current Ai gives only a small increase of collector current Ai Thus a larger fraction of carriers emitted must end up as base current because the increase of collector current is limited by collector current saturation. Through the use of the control circuit of this invention the feedback from collector to base prevents this Ai Further, through the use of the control circuit of this invention a point B on the output characteristic can be selected, and the values of resistors 16 and 17 may be selected with respect to the other parameters in the circuit such that the reverse bias on asymmetric impedance 15 is overcome at this point. In other words the parameters in the circuit and the values of resistors 16 and 17 are so selected that the anode potential of asymmetric impedance 15 is slightly more positive then the potential of the base 3 when the current through the collector as described by the characteristic in Figure 2 reaches a selected point such as point B.

In order to aid in understanding and practicing this invention, the following set of figures and specifications are provided for the inverter circuit of Figure 1, it being understood that no limitations should be implied thereby, it being well established in the art that a wide range of parameters, transistor conductivities and performance characteristics are available for circuits of this type. I

Transistor 1 Germanium P-NP a .972. Resistor 7 2,4K ohms, Resistor 16 15K ohms.

Resistor l7 3.9K ohms.

What has been described is a transistor circuit having applied thereto a control principle wherein an asymmetric impedance is provided connected between a source of potential and the base of an active transistor element and a sensing means is provided for controlling the impedance Resistor 14 of the asymmetric impedance in response to current flow ing in the transistor.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention therefore, to be limited only as indicated by the following claims.

What is claimed is:

l. A non-saturating transistor inverter circuit comprising in combination a transistor having emitter, base and collector connections having the emitter thereof connected to reference potential, a first potential source having one terminal thereof connected to reference potential, a first resistor having one terminal thereof connected to the collector of said transistor and having the remaining terminal connected to the remaining terminal of said first potential source, signal introduction means connected to the base of said transistor and operable to produce current flow therethrough, a second source of potential of opposite polarity to said first source having one terminal thereof connected to reference potential, a diode independent of said signal introduction means connected in collector connection to control the impedance of said diode.

2. A non-saturating transistor inverter circuit comprising a PNP type transistor having emitter, base and collector connections and having said emitter connection connected to reference potential, a first source of potential having the positive terminal thereof connected to reference potential, a first resistor having one terminal connected to said collector connection and having the remaining terminal connected to the negative terminal of said first potential source, an input terminal, a second resistor having one terminal connected to said input terminal and having the remaining terminal thereof connected to said base connection, a first capacitor having one terminal connected to said input terminal and having the remaining terminal thereof connected to said base connection, a second source of potential having the negative terminal thereof connected to reference potential, a third resistor having one terminal thereof connected to the positive terminal of said second potential source and having the remaining terminal thereof connected to said base connection, a diode havingthe cathode thereof connected to said base connection, a fourth resistor having one terminal thereof connected to the anode of said diode and having the remaining terminal thereof connected to said positive terminal of said second potential source,

a fifth resistor having one terminal thereof connected to the anode of said diode and having the remaining terminal thereof connected to said collector connection and an output terminal connected to said collector connection.

3. A non-saturating transistor inverter circuit comprising a NPN type transistor having emitter, base and collector connections and having said emitter connection connected to reference potential, a first source of potential having the negative terminal thereof connected to reference potential, a first resistor having one terminal connected to said collector connection and having the remaining terminal connected to the positive terminal of said first potential source, an input terminal, a second resistor having one terminal connected to said input terminal and having the remaining terminal thereof connected to said base connection, a first capacitor having one terminal connected to said input terminal and having the remaining terminal thereof connected to said base connection, a second source of potential ,having the positive terminal thereof connected to reference potential, a third resistor having one terminal thereof connected to the negative terminal of said second potential source and having the remaining terminal thereof connected to said base connection, a diode having the anode, thereof connected to said base connection, a fourth resistor having one terminal thereof connected to the cathode of said diode and having the remaining terminal thereof connected to said negative terminal of said second potential source, a fifth resistor having one terminal thereof connected to the cathode of said diodetand having the remaining terminal thereof connected to said collector connection and an output terminal connected to said collector connection.

4. A transistor saturation control circuit comprising a transistor having at least emitter base and collector connection thereto, means establishing current flow in the emitter to collector current path of said transistor, a source of potential having a proper polarity when applied to the base connection of said transistor to oppose current flow through said transistor, a control diode, signal introduction means independent of said diode connected to the base of said transistor and operable to influence current flow therethrough, diode impedance control means including a source of potential connected through a voltage divider to a circuit point the potential of which is influenced by the collector current of said transistor, said control means including further means connecting said diode in the low impedance direction between a point in said voltage divider and the base of said transistor whereby the impedance of said diode is controlled in response to the magnitude of said current at said collector, and output signal sensing means responsive to current flow in said transistor.

References Cited in the file of this patent UNITED STATES PATENTS Trousdale Mar. 6, 1956 Priebe et al. Apr. 2, 1957 OTHER REFERENCES ceedingsSymposium on Application of Transistors to Military Electronics Equipment, Ofiice of Sec. of Defense, September 1953, pages 299-321, 

